Manufacture of alkali metal phosphates

ABSTRACT

Alkali metal phosphates can be prepared by adding an alkali metal halide to a solution of phosphoric and nitric acids. The resulting gases can be recovered. After removal of halogen, as by boiling, the solution can be adjusted in nitric acid content and neutralized to yield a fertilizer. Alternatively, the solution can be substantially denitrated, yielding an alkali metal phosphate. Additionally, after denitration the resulting residue can be calcined to yield a water soluble fertilizer. Micronutrients incorporated in the phosphoric acid-nitric acid solution also can be rendered water soluble by this process.

United States Patent 1 1 I Camp, Jr. 1 1 Jan. 2, 1973 1541 MANUFACTUREOF ALKALI METAL 2,888,321 5/1959 Baumann ..25/107 PHOSPHATES 3,554,7291/1971 Curless ..71/36 x 3,375,062 3 1968 C l ..23 102A Inventor; ErnestCamp, 'o Bamngton, 3,010,818 11/1961 1:116: 2: al .71/37 1 3,574,5914/1971 Lyons et a1. ..71 36'x [73] Assignee: Service Company, New. York,FOREIGN PATENTS 0R APPLICATIONS 1,082,963 H1963 Japan ..71/36 1 FlledlOct-28,1970 613,794 1/1961 .Canada ..23/l02A [2]] App]. No; 84,9071,447,996 12/1964 France ..23/102 A Related US. Application Data PrimaryExaminer-Reuben Friedman Assistant Examiner-Richard Barnes [63]Contmuation-tn-part of Ser. No. 725,139, Aprll 2 Anomey j RichardGeaman, Elton R 'Gunn and 1968, Pat. No. 3,563,703, I 4 Joshua J ward[52] U.S.CI. ..71/l,7l/34,7l/35,

, 71/36, 423/309, 423/399 1 1 ABSTRACT [51] int. Cl. ..C05b 7/00 Alkalimetal phosphates can be prepared by adding an [58] Field of Search.....71/35, 36, 1, 34; 23/102 A, alkali metal halide to a solution ofphosphoric and 23/107,203 N; 423/309, 313,399,386 nitric acids. Theresulting gases can be recovered.

After removal of halogen, as by boiling, the solution [56] ReferencesCited can be adjusted in nitric acid content and neutralized to yield afertilizer. Alternatively, the solution can be UNITED STATES PATENTSsubstantially denitrated, yielding an alkali metal 3,579,323 5/1971Gausteretal. ..71/35 S Addilimanyi aftef nitration the 38 3,052,61711/1962 Beekhius 23/102 A residue can be calcined to yleld a watersoluble fertil- 3,600,152 8/1971 Drechseletal ..71/34 izer.Micronutrients incorporated in the phosphoric 3,244,500 5/1966 Stinsonet al. ..7l/34X acid-nitric acid solution also can be rendered water3,414,375 12/1968 Leroy et a1. ..'...23/ 107 X soluble by this process.1 3,347,656 10/1967 Potts et al ..71/36 9/1971 Moore ..23/107 14 Claims,N0 Drawings MANUFACTURE OF ALKALI METAL PHOSPHATES CROSS-REFERENCE TORELATED- APPLICATIONS This application is a continuation in part of mycopending application Ser. No. 725,139 entitled, Improved Process forTreating Phosphate Rock filed Apr. 29, I968, and assigned to the sameassignee as the. present application and now US. Pat. No. 3,563,703.

BACKGROUND OF THE INVENTION This invention relates to an improvement inthe preparation of alkali metal phosphates. There is at present a greatneed for an economical process which will produce alkali metalphosphates, such as potassium phosphates, at a cost sufficiently low topermit their use as fertilizers. Presently, technical grade alkali metalphosphates are made commercially from wet process phosphoric acid usingthe alkali metal hydroxide or carbonate. The naturally occurring alkalimetal compounds, however, such as those of sodium and potassium, aremost commonly mined and recovered in the form of a salt, such as thechloride. Thus, in order to react phosphoric acid with an alkali metalhydroxide or carbonate, the alkali metal salt must first be converted tothe corresponding hydroxide or carbonate. Unfortu-' nately, however, thecost of these converted materials is too expensive for fertilizerproduction. It is thus readily apparent that a process for preparingalkali metal phosphates which employs the alkali metal salt directly,eliminating expensive intermediate processing, would be a highlydesirable and economically attractive process.

The prior art has recognized the desirability of directly producingalkali metal phosphates from such inexpensive compounds as thecorresponding alkali metal salt, such as the chloride. Exemplary of suchprior art are U. S. Pat. No. 2,954,286 to John K. Radley et al. and U.S. Pat. No. 3.0IO,817 to Eugene D. Crittenden. Radley in Pat. No.2,954,268 teaches a process for making a fertilizer which involvesreacting potassium chloride and nitric acid to produce a liquid productcontaining free nitric acid, removing the formed nitrosylchloride andchlorine and treating the product with phosphoric acid. Crittenden in U.S. Pat. No. 3,010,817 teaches that potassium phosphate fertilizers maybe prepared by feeding potassium chloride to'a reactor for reaction withnitric acid and water. Following the reaction, chlorine and nitrosylchloride are separated, leaving an effluent containing mostly potassiumnitrate, nitric acid and water. This effl uent is then fed to a mixingvessel to which sulfuric acid and phosphoric acid are added,-and thismixture is finally supplied to leach phosphate rock.

In each of the above cases the alkali metal salt is first reacted withnitric acid and the reaction product is then combined with phosphate,either by treating with phosphoric acid or by subsequent contact withphosphate rock. The disadvantages of these prior art processes aremanifest when compared with the desirability of directly obtaining thedesired product. In one case, the phosphoric acid must first be formedand thereafter combined with the reaction product of nitric acid andalkali metal salt. In the second case, potassium nitrate is mixed withsulfuric acid and phosphate rock,

low in totalplant food and which contains, as a result of theacidulation of phosphate rock, substantial'amounts of calcium, whichoften is not desirable, One'advantage of the present process is that itprovides an integrated process for the nitric acid acidulation ofphosphate rock, subsequent treatment of the acidulate to remove calcium,the treatment of thecalcium-free solution plant food.

with an alkali metal salt to form the corresponding phosphate, andrecovery or subsequent treatment of said phosphate.

One aspect of this invention relates to the prepar'ation of atetrapotassium pyrophosphate that is totally water-soluble. A commonindustrial method of forming tetrapotassium pyrophosphate is to reactphosphoric acid, either furnace acid or wet process acid, with potassiumhydroxide or potassium carbonate. This reaction yields dipotassiumhydrogen phosphate, which is then heated and dehyrated to give thedesired tetrapotassium pyrophosphate. As mentioned above, this methodinvolves the reaction of the expensive potassium hydroxide or carbonate.Other methods of adding the potassium ion to phosphoric acid involve theaddition of a material such as potassium chloride to phosphoric acid.This mixture, when used as a fertil izer, suffers the disadvantage ofcontaining appreciable amounts of chloride ion. Chloride ion has adeleterious effect on a number of crops, such as tobacco and Idahopotatoes. Also, if the phosphoric acid is wet processv acid, it containscongeneric metal impurities, such as iron and aluminum, that tend toprecipitate when the acid is concentrated. Thus, a chloride-free andwatersoluble high analysis fertilizer is desirable. This inventionoffersa process for making this type of fertilizer.

Another aspect of this invention relates to a method for incorporatingmicronutrients into the fertilizers that can be prepared as describedherein.

The importance of micronutrients (or minor ele-' micronutrients to theoil coating during mixing dispe'rses the trace elements onto thefertilizer granules. By the method described herein for adding .thedesired micronutrients to the nitrate-phosphate mixture, denitrating thesolution and calciningmot-only can the micronutrients be simply anduniformly dispersed throughout the fertilizer, but themicronutrie'ntsare obtained in a substantially water-soluble form, making themcompletely useful and immediately available as SUMMARY OF THE INVENTIONHalide-free alkali metal phosphates can be prepared by reacting amixture of nitric and phosphoric acids with an alkali metal halide. Thehalogen-containing off gases can be separated and recovered, with theresulting solution comprising phosphate, nitrate and alkali metal ions.In one embodiment of this invention, alkali metal'nitrates can berecovered from the solution by crystallization. In another embodiment ofthis invention, excess nitric acid can be evaporated and the remainingsolution neutralized with ammonia to produce a high analysis fertilizer.In still another embodimerit of this invention, substantially allnitrate can be drivenoff and an alkali metal phosphate suitable asfertilizer can be recovered. In yet another embodiment of thisinvention, substantially all nitrate can be removed, the residue can becalcined and a water-soluble alkali metal polyphosphate, useful as afertilizer or a detergent, can be recovered. In still anotherembodiment, the solution comprising phosphate, nitrate and alkali metal'ions can be mixed with a suitable micronutrient or mixture ofmicronutrients, substantially all nitrate can be removed, the residuecan be calcined, and a water-soluble polyphosphate fertilizer containingmicronutrients can be recovered.

4 The solution of nitric and phosphoric acids to which the alkali metalhalide is added can come from any convenient source and is preferablysubstantially free of calcium. In one convenient embodiment, phosphaterock is acidulated with concentrated nitric acid to convert thephosphate rock to an acidulate comprising phosphoric acid, nitric acidand calcium nitrate in solution, as described in my co-pendingapplication Ser. No. 725,139 which is hereby incorporated by reference.Subsequent to acidulation, calcium nitrate is removed from the resultingacidulate. This step is preferably performed by increasing the nitricacid content of the acidulate. Any fluorine compound found in thephosphate rock can, of course, be removed from the acidulate in knownmanner. Following separation of precipitated calcium'nitrate, there isadded to the filtrate, comprising phosphoric acid and nitric acid, anamount of alkali metal salt, such as KCl, necessary to furnish thedesired ratio of M,O:P,O where M is the alkali metal, in the finalproduct. This M O:P,O, ratio can vary from about 0.5;] to about 3:1, sothat from 1 to 6 moles of alkali metal halide can be added per mole of p105. v c i In the acidulation step it is desirable to use concentratednitric acid, preferably having a concentration from about 75 to about 85percent by weight nitric acid. The use of lower concentrations of nitricacid frequently causes the formation of excess gangue,.while digestingwith concentrations of nitricacid greater than about 85 percent proceedsrather slowly and results in losses of phosphoric acid, on the order ofas much as 9 percent. Most preferably, nitric acid of from 80 to 85percentby weight is employed. The temperature of the acidulation is notcritical and can range from about 160 to about 240F. In the acidulationstep, it is desirable to use at least sufficient nitric acid to convertthe phosphate rock to. calcium nitrate and phosphoric acid.Preferably,'- an excess of up to about 15 percent by weight of nitricacid is used, based on the calcium present, calculated as calcium oxide.When employing a 10 to percent excess'of nitric acid, there is attainedan acidulation ratio of from about 2.5 to 2.6 parts by weight of nitricacid per part of calcium calculated as calcium oxide. As previouslyindicated, calcium nitrate is removed from the acidulate, preferably byincreasing the nitric acid content. There are several ways of increasingthe nitric acid'content of the acidulate, including the addition of moreconcentrated nitric acid or the formation of additional nitric acid insitu by the iritroduction of nitrogen oxides and oxygen into theacidulate. Where higher concentration nitric acid, is

added, the nitric acid added is preferably more concentrated than thenitric'acid-used to acidulate the rock. Thus, it is desirable that theadded nitric acid be at least percent by weight l-lNO preferably greaterthan percent and, most preferably, greater than 98% HNO by weight.Alternatively, the nitric acid content in the acidulate can be increasedby introducing one or more nitrogen oxides and oxygen into theacidulate. In a parrapidly. Following separation, the calcium nitratecake can be washed, dried and used in. any convenient manner. Thesupernatant liquid remaining after removal of the precipitated calciumnitrate is the preferred source of the solution of nitric and phosphoricacids to which thealkali metal halide is added.

The alkali metal cations which are useful within the scope of thisinvention can be selected from among lithium, sodium, potassium,rubidium and cesium, with sodium and potassium compounds being preferredbecause they are commercially available-and relatively inexpensive.Potassium halides are particularly preferred because potassium is one-ofthe necessary macronutrients, and its incorporation into a mixturecontaining available nitrogen and phosphorous makes a completefertilizer available. The halide anions useful in this invention can beselected fromamong fluorine, bromine, chlorine and iodine compounds.Since alkali metal chloride compounds are commercially available andrelatively inexpensive, they are preferred. Thus, potassium chloride isthe particularly preferred alkali metal halide. The alkali metal halideemployed can come from any of the usually available salts and can be ofhigh or low purity. The alkali metal halide-can be used in a solid form,preferably finely divided, or as a solution,as desired.

The reacting components can be combined and mixed in anyconvenientmanner. It is often preferred to add the alkali metal halideto the mixed acids at a temperature below about 60C., since there ispractically 'no reaction at or below this temperature level. Thus, theslurry can be introduced into a closed system without giving off .fumesthat would'be evolved at a higher temperature. The gases evolvedas aresult of the reaction with the preferred alkali metal chloride comprisechlorine, nitrogen dioxide and nitrosyl chloride.

can be passed through concentrated nitric acid to accomplish saidoxidation. The resultant gases, comprising nitrogen dioxide andchlorine, can be recovered and readily separated by well-knownfractionation techniques. The solution remaining can be treated inseveral ways.

In one embodiment alkali metal nitrate can be prepared by removingexcess nitric acid from the above solution, as by boiling, andthereafter crystallizing the alkali metal nitrate from the solution.

In still another embodiment, the solution comprising alkali metal ions,nitric acid and phosphoric acid can be substantially denitrated, as byboiling. The remaining phosphoric acid solution can be appropriatelyneutralized, such as by the addition of ammonia, to yield asubstantially chloride-free, high analysis mixed fertilizer. Byadjusting the amounts of nitric acid, phosphoric acid, potassiumchloride and ammonia, fertilizers having varying ratios of N-PK can beobtained.

In still another embodiment, a metaphosphate-type fertilizer can beprepared. To the solution comprising nitric acid and phosphoric acid isadded sufficient alkali metal halide to obtain an M O:P O ratio (where Mis the alkali metal) of approximately 1:1 which is characteristic ofalkali metal metaphosphate. Excess nitrate can be driven off, as byboiling, and the residue, comprising alkali metal dihydrogen phosphate,is calcined to give a product comprising alkali metal metaphosphate.

In another embodiment the solution can be substantially denitrated, asbyboiling, and the residue can be calcined at a temperature of from about650C. to about 800C., for a period of from about 1 hour to about 2hours, to form an alkali metal polyphosphate. In this embodiment, aratio of M O:P O of about 2:I (where M is the alkali metal) is needed toyield a polyphosphate such as M P O The conditions for calcining arewell-known. The polyphosphates formed are substantially completelywater-soluble, even in the presence of iron and aluminum congenericmetal impurities found in the phosphoric acid derived from the phosphaterock. Thus, this process has the advantages of making substantiallychloride-free, substantially completely water-soluble high analysis P-Kfertilizer. The product is quite desirable and is less expensive thanpolyphosphates from other processes. I

In still another embodiment, micronutrients can be incorporated into thesolution of phosphate, nitrate and alkali metal ions that remains afterthe removal of nitrogen dioxide and chlorine gases. The micronutrientsmost usually incorporated in fertilizers are boron, zinc, iron andmanganese. Copper and molybdenum are less frequently used becausedeficiencies of these metals are usually less widespread than one thoseof the micronutrients described above. The following minimum percentagesby weight of the more frequently employed micronutrients have beenadopted as standard by some states and may be considered as typical ofthe amounts which can be incorporated in a fertilizer:

B 0.02 Fe 0.10 Mn 0.05 Zn 0.05

That the micronutrient content must be carefully controlled can be seenfrom the fact that the range between inadequate and toxic amounts ofmany micronutrients is fairly small. The principal micronutrients areusually added in the form of borax and the sulfates of the other metals,although other compounds can be used. In accordance with the embodimentdescribed above, the micronutrients, for example, in the form ofpowdered borax and sulfates of the metals, can be added to the acidsolution in sufficient amount to satisfy the proposed minimumrequirements of the locale in which they are to be used, thereaftersubstantially denitrating the resulting solution, as by boiling, andthereafter calcining the residue at a temperature and for a period oftime sufficient to form an alkali metal polyphosphate matrix containingthe micronutrients. The calcined product is substantially completelywater soluble.

In still another embodiment, the fertilizer comprising a mixture ofmicronutrients dispersed in the alkali 7 metal phosphate fertilizerbase, prepared as described above, can be combined or mixed withthecomplete fertilizer formed by the ammoniation of the alkali metalphosphate base. By proper adjustment and manipulation of the variouscomponents, a commercially valuable complete fertilizer containingmicronutrients can readily be obtained, with the kind and concentrationof micronutrients, as well as the N-P-K ratio, being variable upondemand.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE I Phosphate rock,containing approximately 30 percent by weight P O and approximately 47percent by weight calcium (measured as calcium oxide) was crushed to aparticle size that substantially passed through a 30 Tyler Mesh screen.Nitric acid was then added to stoichiometric crushed rock, with the acidconcentration being about percent by weight and the amount of acid beingabout 10 percent in excess of the stiochiometric amount needed toproduce calcium nitrate and phosphoric acid. After the phosphate rockwas digested for about 2 hours at a temperature of about 160F.,concentrated nitric acid (approximately. percent by weight l-INO wasadded in an amount such that the water content'of the supernatantsolution was less than about 14 percent by weight. The calcium nitratewhich precipitated was'removed by filtration. The filtrate analyzed62.33% N 0 8.55% P 0 and 0.53% CaO. Potassium chloride was added to thisfiltrate so as to furnish a 1:1 ratio of K O:P O in the final product.The potassium chloride-nitric acid-phosphoric acid mixture was thenheated to boiling, evolving halogen-containing off-gases. The solutionresulting from this treatment comprised phosphate, nitrate, potassiumand hydrogen ions. Excess nitric acid was evaporated from the solutionuntil the residue was substantially a solution of potassium nitrate inphosphoric acid. Cooling of this solution resulted in the precipitationof crystalline potassium nitrate.

EXAMPLE u solution of potassium, hydrogen, nitrate and phosphate ionsprepared as described in Example I was denitrated by evaporation todryness, resulting in a residue comprising KI-I PO Calcining of thisresidue at 425C. for a period of about 1 hour gave a product comprisingKPO and analyzing 37.78% K 0, 50.2% P 0, and 1.15% N, or about 89% totalplant food.

EXAMPLE III Water-soluble fertilizers were prepared as follows: To aportion of the filtrate solution obtained as described in Example I wasadded KCl to provide about 2 moles K per mole P 0 The off-gases wereremoved and the solution was concentrated by boiling. This solution wasevaporated to dryness and the residue heated to 740C. for one-half hour.The product analyzed 40.1% P 0 55.54% K 0 and 0.01% nitrogen and wassubstantially completely water soluble, in spite of the presence of suchcongeneric metals as iron and aluminum. X-ray diffraction confirmed itsstructure as substantially K P O EXAMPLE IV A water-soluble fertilizercontaining soluble micronutrients can be prepared as follows:

To a portion of the filtrate solution of nitric and phosphoric acidsprepared'as described in Example I is added sufficient KCI to provide aK O:P O ratio of 2: l To this solution, after removal of volatile gases,are added the micronutrient compounds. Thus, to suffrcient filtratesolution to provide one hundred lbs., dry basis, of potassium phosphate,are added a powdered mixture of 0.178 lb. borax, 0.203 lb. manganoussulfate tetrahydrate and 0.22 lb. zinc sulfate heptahydrate. Afterboiling until dry and calcining at 740C. for one hour, a water-solubleproduct isobtained.

When wet process phosphoric acid is used, as is the casein the aboveexamples, the addition of iron, in the form of ferrous sulfate, isgenerally unnecessary insofar as the wet process acid often containsmore iron, calculated as F6 0 A fertilizer containing micronutrientsprepared as described above can be applied directly or can be mixed witha fertilizer not containing micronutrients and blended so as to obtain afertilizer whose properties can be tailored for a specific use.

Iclaim: I

l. A method for making substantially halide-free metal phosphates whichcomprises;

I a. reacting an alkali metal halide with a mixture of nitric andphosphoric acids in which the concentration of said mixture is fromabout 55 percent wt. to about 70 percent wt. nitric acid and less thanabout 14 percent water,

b. separating the halogen-containing off-gases, and

1 percent or such' 3. A method according to claim 2 in which atleastpart of the halogen-containing off-gases are recycled throu h thesolution. 4. method according to claim 1 m which the alkali metal halideis added in an amount such that the resultant M O:P O mole ratio, whereM is alkali metal, lies in the range of from about 0.521 to about 3: 1.

5. A method according to claim 1 in which the alkali metal halide ispotassium chloride.

6. A method according to claim 1 in which the solution recovered isheated to evaporate excess nitric acid and then neutralized with ammoniato produce a substantially halide-free fertilizer.

7. A method according to claim 1 in which the solution recovered istreated to drive off substantially all nitrates, and an alkali metalphosphate residue is recovered.

8. A method according to claim 7 in which the phosphate residuerecovered comprises about 89 percent total plant food. I

9. A method according to claim [in which the solution recovered in step(c) is treated to remove halogen values and nitrates and then calcinedto convert at least part of the phosphate present to the polyphosphateform.

10. A method according to claim 9inwhich the al- I 12. A methodaccording to claim 1 in which plant micronutrients are added to thesolution recovered in step (c), the solution is heated to remove halogenvalues, excess water and nitrates, and the residue obtained thereby iscalcined to convert at least partjof the phosphate therein topolyphosphate. I

13. A method according to claim 12 in which the alkali metal halide ispotassium chloride, added in an amount of from about two to about sixmoles of KCl per mole of P 0 the solution is boiled to remove water,chlorine values and nitrates, and the residue is calcined at atemperature of from about 650C. to

about 800C. for a period of from about one-half to about 2 hours. I

14. A method according to claim 13 in which the product obtained ischaracterized by substantially complete water solubility.

2. A method according to claim 1 in which the nitric acid concentrationis sufficient to oxidize at least part of the halide reaction productsto free halogen.
 3. A method according to claim 2 in which at least partof the halogen-containing off-gases are recycled through the solution.4. A method according to claim 1 in which the alkali metal halide isadded in an amount such that the resultant M2O:P2O5 mole ratio, where Mis alkali metal, lies in the range of from about 0.5:1 to about 3:1. 5.A method according to claim 1 in which the alkali metal halide ispotassium chloride.
 6. A method according to claim 1 in which thesolution recovered is heated to evaporate excess nitric acid and thenneutralized with ammonia to produce a substantially halide-freefertilizer.
 7. A method according to claim 1 in which the solutionrecovered is treated to drive off substantially all nitrates, and analkali metal phosphate residue is recovered.
 8. A method according toclaim 7 in which the phosphate residue recovered comprises about 89percent total plant food.
 9. A method according to claim 1 in which thesolution recovered in step (c) is treated to remove halogen values andnitrates and then calcined to convert at least part of the phosphatepresent to the polyphosphate form.
 10. A method according to claim 9 inwhich the alkali metal halide is potassium chloride, added in an amountof from about 2 to about 6 moles of KCl per mole of P2O5, the solutionis boiled to remove chloride and water and the residue obtained therebyis calcined at a temperature of about 740*C. for from 1 to 2 hours. 11.A method according to claim 10 in which the calcined product ischaracterized by substantial freedom from chloride, nitrate and calciumand by being substantially water soluble.
 12. A method according toclaim 1 in which plant micronutrients are added to the solutionrecovered in step (c), the solution is heated to remove halogen values,excess water and nitrates, and the residue obtained thereby is calcinedto convert at least part of the phosphate therein to polyphosphate. 13.A method according to claim 12 in which the alkali metal halide ispotassium chloride, added in an amount of from about two to about sixmoles of KCl per mole of P2O5, the solution is boiled to remove water,chlorine values and nitrates, and the residue is calcined at atemperature of from about 650*C. to about 800*C. for a period of fromabout one-half to about 2 hours.
 14. A method according to claim 13 inwhich the product obtained is characterized by substantially completewater solubility.